Saturday, March 3, 2012

Guest blog #25: Ashwin Salvi on greenhouse gases and reforestation

Here are some thoughts from Ashwin, a previous guest blogger, on reforestation. I appreciate the post, particularly because its nature is different than what is typically written about on the blog.

Over the past few years, there has been a lot of talk about carbon dioxide (CO2) sequestration to combat climate change. Sequestration is, in a nutshell, the capturing of airborne CO2, which has a warming effect on the climate, and storing it in liquid or solid form, either underground or on the surface. Darshan has written about such geo-engineering approaches and the ethical and procedural justice issues surrounding them previously. Today, I want to focus a bit more on the technical aspects of sequestration.

A recent Michigan Energy Club lecture got me thinking about CO2 sequestration via reforestation to reduce the presence of CO2 in the atmosphere. While replanting trees is a good idea, the issues are a bit more complicated once they are unpacked a bit.

Firstly, it is important to note that growing trees does absorb a considerable amount of CO2 from the atmosphere. It is estimated that that global forests absorb ~20% of CO2 emitted from fossil fuel combustion. However, all living things respire; trees also emit CO2 via respiration! While the amount of CO2 released through respiration is less than the CO2 absorbed for their growth, the point is that we cannot forget that the CO2 and trees is not a one-way operation. In addition and possibly more significant, trees also emit a variety of hydrocarbons (HC) that can lead to increased tropospheric ozone levels. Of course, these biogenic emissions are completely natural. (Click here to read about an interesting study comparing HC emissions from different trees.) It is what is anthropogenic that is of deeper concern.

Furthermore, it also matters where reforestation takes place. Trees growing in tropical climates are more effective at absorbing CO2 than those growing in higher latitude forests. Higher latitude forests have actually been seen to produce a net warming effect on the climate. The darker leaves of these trees absorb more heat and outweigh the cooling effect CO2 absorption and evapotranspiration. This is because the albedo, or reflectivity, of the earth’s surface changes from a higher value (with snow), to a lower value (darker leaves), making less of the incident solar radiation reflect back into space. In addition, higher latitude forests experience seasonal effects, reducing their ability to absorb CO2 due to tree hibernation.

Tropical forests are seen to be more effective at CO2 absorption due to faster growth rates stemming from year round growth, abundant sunlight and rain. In addition, evapotranspiration from the leaves of trees also contribute to a net cooling effect.

Let’s think further down the line, though, toward the end of the tree’s life. The tree spent its entire life absorbing carbon from the atmosphere and now that the tree is dead, where does the carbon go? Well, as the tree decomposes, the carbon goes back into the atmosphere as CO2 and methane, with methane being one hundred times more potent a greenhouse gas than CO2. So while, the tree does a great job taking carbon out of the atmosphere while it is alive, the problem of greenhouse gases in the atmosphere really isn't solved, but just kicked down the road; the tree is just an ephemeral holding box. Therefore, trees as a means of CO2 sequestration will help in the shorter term, but a longer term solution (like reduction of emitted CO2) must be what is tackled.

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